1. Field of the Invention
The present invention relates to a microporous high density polyethylene film and a method of producing the same. More particularly, the present invention pertains to a microporous high density polyethylene film, which has a high productivity as well as a superior extrusion-compoundability and stretchability, and which can improve a performance and stability of a battery produced using the same, and a method of producing the same.
2. Description of the Prior Art
Having chemical stability and superior physical properties, a microporous polyolefin film is widely used as various battery separators, filters, and ultrafiltration membranes.
The production of the microporous film using polyolefin may be conducted according to the following three processes. In a first process, polyolefin is processed into a thin fiber to produce a nonwoven fabric-shaped microporous film, a second process is a dry process, in which a thick polyolefin film is prepared and stretched at low temperatures to create micro cracks between lamellas corresponding to a crystalline portion of polyolefin to form micro pores in polyolefin, and a third process is a wet process, in which polyolefin is compounded with a diluent at high temperatures to form a single phase, phase separation of polyolefin and diluent is initiated in a cooling step, and the diluent is extracted to form pores in polyolefin. In comparison with the first and second processes, the wet process, corresponding to the third process, produces a relatively thin microporous film with uniform thickness and excellent physical properties, and thus, the microporous film according to the wet process is widely used for an isolation membrane of a secondary battery, such as a lithium ion battery.
A method of producing a porous film according to a wet process is disclosed in U.S. Pat. No. 4,247,498, which comprises blending polyethylene and a compatible liquid with each other at high temperatures to form a thermodynamically homogeneous solution, and cooling the solution to initiate solid/liquid or liquid/liquid phase separation, thereby producing the porous polyolefin film.
U.S. Pat. No. 4,335,193 discloses a technology of producing a porous polyolefin film, which includes blending polyolefin, an organic liquid, such as dioctylphthalate and liquid paraffin, and inorganic filler; forming the blend; and removing the organic liquid and inorganic filler from the formed blend, which is also indicated by U.S. Pat. No. 5,641,565. However, the technology is disadvantageous in that the inorganic filler, such as silica, is used in compounding process, it is difficult to conduct feeding and compounding processes of the inorganic filler, and a subsequent process of extracting and removing the inorganic filler must be additionally conducted, and thus, the technology becomes very complicated and also it is difficult to increase a stretching ratio.
U.S. Pat. No. 4,539,256 recites a basic method of producing a microporous film, which includes extrusion molding a mixture of polyethylene and a compatible liquid, stretching the formed mixture, and extracting the compatible liquid from the stretched mixture.
In conjunction with the earnest use of a secondary battery, efforts have been continuously made to improve the productivity and physical properties of a microporous film. A representative example is to improve the strength of the microporous film by using ultra-high molecular weight polyolefin (UHMWPO) with a weight average molecular weight of about 1,000,000, or blending such a UHMWPO with a composition to increase a molecular weight of the composition.
With respect to this, U.S. Pat. Nos. 4,588,633 and 4,873,034 suggest a process of producing a microporous film, in which polyolefin with a weight average molecular weight of 500,000 or more and a diluent capable of dissolving polyolefin at high temperatures are subjected to two step solvent extraction and stretching steps. However, this process is disadvantageous in that in order to improve a poor compoundability of UHMWPO with the diluent and a poor extrudability of UHMWPO, which are considered as disadvantages of UHMWPO, an excessive amount of diluent is used in an extruding step, and the diluent must be extracted through two steps, before and after stretching.
U.S. Pat. No. 5,051,183 introduces a microporous polyolefin film, which includes a composition with a polydispersity index (weight average molecular weight/number average molecular weight) of 10-300, containing 10-50 wt % polyolefin having 1% or more UHMWPO with a weight average molecular weight of 700,000 or more, and 90-50 wt % solvent, such as a mineral oil. In this regard, the composition is extruded to form a gel-like sheet and stretched at a temperature range from a melting point of the composition to a temperature higher than the melting point by 10° C., and the solvent is then extracted from the composition, thereby forming the porous film. However, the above patent is disadvantageous in that a molecular weight distribution is broadened and the composition contains an excessive amount of polyolefin with a high molecular weight because the composition is blended with UHMWPO. In such a case, chain entanglement intensely occurs due to polyolefin molecules, leading to significantly reduced stretchability. In other words, a breakage phenomenon occurs at a high stretching ratio and speed, and a non-uniform stretching phenomenon caused by local incomplete stretching occurs at a low stretching ratio.
To avoid the above disadvantages, it is necessary to increase a stretching temperature to make the composition soft during a stretching process, or to reduce the stretching speed to gain the same effect as an increase of a temperature of the composition. In such a case, however, orientation of a resin is poor during the stretching process to lower a stretching effect, thereby reducing physical properties of the end porous film. Furthermore, a film made of the resin with a broad molecular weight distribution has more defects, caused by molecules with a relatively low molecular weight, than a film made of the resin with low molecular weight molecules, thus having reduced impact and puncture strengths. Like this, when the microporous film has the broad molecular weight distribution, the puncture strength, which is one of the most important physical properties of the microporous film, is poor. In other words, UHMWPO added into the microporous film does not contribute to improving the physical properties of the microporous film. The above disadvantages are also found in relating prior arts, for example, Japanese Pat. Laid-Open Publication Nos. Hei. 06-234876 and Hei. 06-212006, and U.S. Pat. No. 5,786,396.
Meanwhile, Japanese Pat. Laid-Open Publication No. Hei. 09-3228 suggests a method of improving physical properties of a microporous film by balancing stretching ratios in a machine direction (MD) and a transverse direction (TD) using the composition similar to that of the above patent.
Japanese Pat. Laid-Open Publication No. Hei. 09-259858 provides a method of producing a microporous polyethylene film, which comprises preparing a solution, including 10-80 wt % resin composition, containing 70-99 wt % polyethylene with a weight average molecular weight of 500,000 or more and 1-30 wt % low molecular weight polyethylene with a weight average molecular weight of 1,000-4,000, and 20-90 wt % solvent; extruding and cooling the solution using a die to form a gelatinized composition; stretching the gelatinized composition; and extracting the remaining solvent from the stretched composition, so as to reduce a shutdown temperature (at which the microporous film is molten to block pores to shut out an electric current, and thus ignition and explosion are prevented when a temperature of a battery increases due to an abnormal operation of the battery) of the microporous polyethylene film. In this respect, this method is characterized in that low molecular weight polyethylene with the weight average molecular weight of 1,000-4,000 is used to reduce the shutdown temperature of the microporous film. However, this method has two problems. First, use of a molecule with a low molecular weight brings about a reduction of the molecular weight and an increase of a molecular weight distribution, thereby reducing physical properties of the microporous polyethylene film. From examples of the patent, it can be seen that a tensile strength of the microporous polyethylene film is a relatively low 1,000-1,200 kg/cm2. Second, a sophisticated technology is required to compound polyolefin and a diluent or solvent. In such a case, commercially, it is necessary to use a twin screw extruder, a kneader, a Banbury mixer and the like. As well, as described above, in case that resins having largely different viscosities from each other (ultra high molecular weight polyethylene with the weight average molecular weight of 500,000 or more and low molecular weight polyethylene with the weight average molecular weight of 1,000-4,000) are blended with the solvent, there are present problems in the course of compounding the resin and solvent, and in the course of compounding the two resins having the different molecular weights from each other (they have largely different viscosities from each other in a molten state). In such a case, fine gels or fish eyes may occur in the end film, thereby reducing a quality of the film. To avoid the fine gel or fish eye, there may be provided a method of increasing a residence time of a melt in an extruder, but the method has a disadvantage of the reduced productivity.
U.S. Pat. No. 5,830,554 recites a method of producing microporous polyolefin film, in which a solution, containing 5-50 wt % resin with a weight average molecular weight of 500,000-2,500,000 and a ratio of the weight average molecular weight to a number average molecular weight of 10 or less, is subjected to extruding, stretching, and extracting steps. According to the method, a large amount (preferably, 80-90 wt %) of solvent is used to avoid a problem regarding nonuniform extrusion caused by an increased viscosity of the resin in the course of extruding the ultra high molecular weight resin, and thus, a porosity is increased and a tensile strength of the porous film becomes 800 kg/cm2 or more (in examples, 950-1,200 kg/cm2), which means that the physical properties of the porous film are not significantly improved.
Furthermore, U.S. Pat. No. 6,566,012 discloses a method of producing a microporous polyolefin film usefully applied to a battery separator, in which 10-40 wt % ultra high molecular weight polyolefin with a weight average molecular weight of 500,000 or more, or 10-40 wt % a resin composition containing ultra high molecular weight polyolefin with a weight average molecular weight of 500,000 or more, and 90-60 wt % solvent are subjected to extruding, molding, stretching, extracting, and heat-setting steps.
As described above, the prior arts employ the resin with the high molecular weight to improve the physical properties of the porous film, but an increase of the molecular weight of the resin may bring about problems, such as an increased extrusion load, a poor extrusion-compoundability of the resin with the solvent, an increased load of a stretcher during a stretching process, occurrence of non-uniform, and a reduced productivity due to a decrease of a stretching speed and ratio.
The present inventors have conducted extensive studies to avoid the above disadvantages occurring in the prior arts, resulting in the finding that defects of polyethylene can be prevented from being formed by controlling contents of low molecular weight polyethylene molecules contained in polyethylene to a predetermined level or less, thereby accomplishing the present invention.